Process and apparatus for removal of photoresist from semiconductor wafers

ABSTRACT

A process for removing photoresist from semiconductor wafers is disclosed wherein pressure in excess of one atmosphere is applied to ozone, followed by a mixing of the ozone with deionized water via a series of nozzles, and finally where the semiconductor wafers having at least one layer of photoresist are exposed to the mixture of ozone and deionized water. The temperature during the process is maintained at above ambient temperatures of 20-21° C. or 70° F.  
     An apparatus for the removal of photoresist from semiconductor wafers wherein the apparatus is comprised of a tank capable of holding semiconductor wafers, a series of nozzles, a source of ozone connected to the tank, a source of deionized water connected to the tank; and finally a means for recirculating the deionized water connected to the tank

FIELD OF THE INVENTION

[0001] This invention relates to methods and systems for removing photoresist from the surfaces of silicon semiconductor wafers.

BACKGROUND OF THE INVENTION

[0002] The need for quick and efficient removal of photoresist is critical in the area of semiconductor manufacturing. In order to produce a useful semiconductor wafer, first a silicon crystal is grown, sliced into thin wafers, and exposed to a photoresist which forms a layer on the wafers. Multiple layers of photoresist can be formed on the surface of the wafers and then etched off to form patterns on the wafers.

[0003] The use of DIO₃ which is a mixture of chilled distilled water (DI) and ozone (0 ₃), to remove photoresist from surfaces of a silicon wafer has been taught by Matthews in U. S. Pat. No. 5,776,296. Matthews discloses a process and an apparatus for removing photoresist from a semiconductor wafer using DIO₃ at sub-ambient temperatures of 1 to 15° C. wherein the ozone is introduced into the process tank with “a composite element having a permeable member and a nonpermeable member, the permeable member having a top portion and a bottom portion, a means defining an open space in a center portion of the permeable member, and a means defining a trench positioned on the top portion of the permeable member between an outer periphery of the permeable member and the means defining an open space.” The Matthews system suffers from certain disadvantages with respect to the speed, efficiency, and effectiveness of photoresist removal.

[0004] It is an object of the present invention to provide an improved process of removal of photoresist from semiconductor wafers during the manufacture thereof. Another object is to provide a process and system at high rates and efficiency.

SUMMARY OF THE INVENTION

[0005] These objects, and others which will become apparant from the following disclosure and the accompanying drawings, are achieved by the present invention which comprises in one aspect a method of removing photoresist from silicon wafers wherein DIO₃ water which is a mixture of deionized (“DI”) water and ozone (“O₃”) is sprayed at ambient temperature or higher in a process tank via nozzles.

[0006] In another aspect, the apparatus of the invention comprises a tank capable of holding semiconductor wafers and one or more nozzles within the tank adapted to spray a mixture of deionized water and ozone.

[0007] Preferably a pressure plenum set in excess of one atmosphere, a temperature control system, an ozonator, a filter connected to the tank, and a recirculating pump are included. It is further preferred that the temperature controller is set to maintain the liquid temperature at 20-21° C. or higher.

[0008] A preferred method comprises placing semiconductor wafers within a process tank which is filled with deionized water, introducing ozone and continuously the ozone by pumping ozone and recirculating it through a filter, through an ozonator and then back to the pressurized plenum connected to the process tank. The ozone is generated from the generator and fed into the tank and also to another ozonator where the ozone gas is mixed with the deionized water. In addition, the ozone is mixed with deionized water which is sprayed onto the silicon semiconductor wafers via nozzles. The DIO₃ water mixture can be exposed to the photoresist in the form of a fog and/or tiny droplets of water. The concentration level of the gaseous and dissolved ozone can be monitored using inline ozone analyzers.

[0009] Agitation of the DIO₃ water on the photoresist layers raises the rate of photoresist removal, i.e., the “strip rate” for photoresist treated with DIO₃ water is linked to the velocity rate of the DIO₃ water. Notably, an increase in the fluid velocity reduces the boundary layer thickness, thereby resulting in a higher rate of O₃ oxidizing the photoresist (also known as “the etching rate”). In addition, the use of sonic energy also reduces the boundary layer thickness, again resulting in a higher rate of O₃ oxidizing the photoresist or etch rate. Thus, the higher the kinetic energy and O₃ concentration, the shorter the strip time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic of a photoresist stripping apparatus comprising nozzles according to the invention.

[0011]FIG. 2 is a chart of the rate of etching of the photoresist versus the ambient temperature.

[0012]FIG. 3 is a chart of the rate of etching of the photoresist versus the velocity of DIO₃ water.

DETAILED DESCRIPTION OF THE INVENTION

[0013] Inasmuch as the etch rate of photoresist utilizing a solution of O₃ in DI water increases linearly with the increase in O₃ concentration, the object of the present invention is to provide a method which significantly increases the O₃ concentration in a DI water solution from the methods currently available. In addition, the series of nozzles seek to increase the velocity rate of the DIO₃ water so as to reduce the boundary layer thickness and therefore increase the rate of etching.

[0014] Thus, it is an object of the invention to provide for a process for removing photoresist from semiconductor wafers comprising applying pressure in excess of one atmosphere to ozone, mixing said ozone with deionized water, and finally exposing the semiconductor wafers having at least one layer of photoresist to said mixture of ozone and deionized water via a series of nozzles.

[0015] The preferred process for removing photoresist from semiconductor wafers includes recirculating the mixture of deionized water and ozone and adding ozone when needed so that the concentration of ozone in the mixture is about constant. The temperature of the liquied in the process tank is maintained at or above ambient temperature, which is about 20-21° C. The mixture of deionized water and ozone is agitated by spraying the semiconductor wafers with droplets of the mixture of deionized water and ozone.

[0016] The apparatus includes the process tank capable of holding semiconductor wafers, one or a series of nozzles, a source of ozone connected to the tank, a source of deionized water connected to the tank, and means for recirculating the deionized water. In some preferred embodiments a pressure plenum connected to the source of ozone is included, and the series of nozzles is located at the top of the tank. The means for recirculating the deionized water is preferably connected to the source of the ozone and a pressure plenum.

[0017] In some embodiments, the apparatus includes cassettes filled with semiconductor wafers having a layer or multi-layers of photoresist wherein the semiconductor wafers are exposed to pressurized DIO₃ water at ambient temperatures and with a velocity produced by a series of nozzles so as to etch or remove the photoresist at a higher rate than previously known.

[0018] Referring now to FIG. 1, the illustrated photoresist removal apparatus including a process tank 10 which holds semiconductor wafers 20 in a cassette 30. The semiconductor wafers 20 have a layer or multiple layers of photoresist baked onto them. The semiconductor wafers 20 are placed within the cassette 30 and process tank 10 so as to have this photoresist removed in as quickly and completely as possible.

[0019] The semiconductor wafers 20 are thus exposed to DIO₃ water 40 at ambient temperatures of between 20-21° C. DIO₃ water 40 is produced by combining pure ozone gas (O₃) 50 from an ozone generator 60 with deionized water 70. Deionized water 70 is pumped into the process tank via multiple nozzles 80. The multiple nozzles 80 thus produce DIO₃ fog wherein the DIO₃ fog interacts with the photoresist on the semiconductor wafers 20. The multiple nozzles 80 also produce DIO₃ droplet varying in diameter which then interact with the photoresist on the semiconductor wafers 20. The droplet size of the sprayed deionized-water will range from a few microns in the fogging stage to a few millimeters in size once collected on the semiconductor wafers 20. The level of ozone in the DIO₃ water 40 is kept in constant through regulation of a gas O₃ sensor 90. If the ozone level is high enough, it passes through the gas sensor 90 and into the process tank 10. On the other hand, if the ozone level is too low, then the ozone passes through the gas sensor 90 and into an ozonator 100 where more ozone is added until it reaches the proper level, at which time it passes into the process tank 10. O₃ 50 is dispersed into the process tank 10 via the multiple nozzles 90 and thus the process begins anew.

[0020] The DIO₃ water 40 flows from the process tank 10 back into a pump 120 after passing through a second sensor, a dissolved O₃ 50 sensor 130 which measures the level of concentration of O₃ 50 in the DIO₃ water 40. The recirculated DIO₃ water 40 passes through the pump 120 and into the filter 140 before O₃ 50 added back into the DIO₃ water 40.

[0021] The temperature in the process tank 10 is maintained at higher than the atmospheric pressure to help maintain a high ozone concentration inside the process tank 10 and to enhance the stripping rate. Further, since the process tank 10 is kept pressurized the temperature within the process tank 10 is increased above ambient temperature.

[0022] Upon condensation of the DIO₃ water 40 as droplets upon the semiconductor wafers 20 the DIO₃ water 40 is collected in the bottom 150 of the process tank 10. This liquid is then recirculated through the pump 120 and process through a filter 140. The DIO₃ water 40 is then passed through the gas sensor 60 at which time the ozone layer is measured and if low passes through the ozonator 100 where more ozone is added until it reaches the proper level, at which time it passes into the process tank 10.

[0023] Referring now to FIG. 2, the rate relationship between the ambient temperature and concentration of ozone in the DIO₃ water 40 indicates that a process time of 15-25 min. can be used to strip about 15000 Angstrom of positive hard baked photoresist at ambient temperature. The photoresist strip rate depends on the dissolved O₃ concentration and average fluid velocity.

[0024] Referring now to FIG. 3 which presents experimental data showing the relationship between the etching rate and the velocity of DIO₃ water 40, the higher the kinetic energy (from the fluid velocities) and ozone concentration, the shorter the strip time. By increasing the fluid velocity and turbulence intensity, ozone is introduced to the wafer surface and penetrates the boundary layer. The series of nozzles play a significant role to reduce the process time significantly when optimized. The removal rate has shown to depend on the fluid velocity, turbulence intensity, and ozone concentration.

[0025] Further, the pressure of the DIO₃ water also directly affects the O₃ concentration and correspondingly affects the etch rate. In summary, the etch rate is affected by the O₃ concentration in the DI water which is in turn affected by the temperature and pressure of the DIO₃ water. Further, the etch rate is directly affected by the velocity rate of the DIO₃ water.

[0026] Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. 

What is claimed is:
 1. A process for removing photoresist from semiconductor wafers comprising applying pressure in excess of one atmosphere to ozone; mixing said ozone with deionized water; and exposing semiconductor wafers having at least one layer of photoresist to said mixture of ozone and deionized water via at least one nozzle.
 2. The process according to claim 1 further comprising the step of placing the semiconductor wafers within a processing tank.
 3. The process according to claim 1 further comprising the step of keeping the temperature in the processing tank at ambient temperature.
 4. The process according to claim 3 wherein the temperature is above 20-21° C.
 5. The process according to claim 1 wherein the mixture of ozone and deionized water is recirculated and flows back into the processing tank.
 6. The process according to claim 1 wherein the mixture of deionized water and ozone is recirculated and ozone added thereby keeping the concentration of ozone in said mixture about constant.
 7. The process according to claim 7 wherein said mixture of deionized water and ozone is agitated via at least one nozzle.
 8. The process according to claim 1 where said mixture of deionized water and ozone is a vapor within said processing tank.
 9. The process according to claim 2 wherein said mixture of deionized water and ozone is pumped into said processing tank in droplets.
 11. An apparatus for the removal of photoresist from semiconductor wafers, comprising: (a) a tank capable of holding semiconductor wafers; (b) at least one nozzle set within said tank; (c) a source of ozone connected to said tank; (d) a source of deionized water connected to said tank; and (e) a means for recirculating said deionized water.
 12. An apparatus according to claim 11 further comprising a pressure plenum connected to said source of ozone.
 13. The apparatus according to claim 11 wherein said pressure plenum is set in excess of one atmosphere.
 14. An apparatus according to claim 11 further comprising a means for temperature control.
 15. An apparatus according to claim 14 wherein said temperature is above 20-21° C.
 16. An apparatus according to claim 11 wherein said means for recirculating said deionized water is connected to said source of ozone.
 17. An apparatus according to claim 11 wherein said means for recirculating said deionized water is connected to a pressure plenum.
 18. The apparatus according to claim 11 further comprising an ozonator.
 19. The apparatus according to claim 11 further comprising a filter connected to said processing tank.
 20. The apparatus according to claim 11 wherein said source of ozone is an ozone generator.
 21. The apparatus according to claim 11 wherein said means for recirculating said deionized water is a pump.
 22. The apparatus according to claim 11 wherein the nozzles are at the top of the process tank. 